The present invention relates to the allocating of communication resources in a space division multiple access radio system. It relates more particularly to the allocating of codes in such a system.
Certain communication systems use codes consisting of binary sequences known both to a sender and to a receiver and combined with a digital signal to be transmitted, so as to allow only the receiver to which the sequence is addressed to extract this digital signal from a set of signals received. This operation is made possible by the use of codes having certain correlation properties. Such is the case for example for multiple access systems of CDMA (“Code Division Multiple Access”) type.
The method is applied in particular to the allocating of orthogonal codes with variable spreading factor (OVSF, “Orthogonal Variable Spreading Factor”) that are used in certain CDMA systems such as UMTS (“Universal Mobile Telecommunication System”). These OVSF codes are chosen from a set of codes of the type of the tree represented in FIG. 1. Each code cSF,i (1≦i≦SF) is a sequence of SF samples called “chips”, each taking the value ±1, with SF=2L−k, L being a positive integer (equal to 8 in the case of UMTS) and k an integer variable such that 0≦k≦L. The tree is defined by:                c1,1=(1),        c2.SF,2i-1=(cSF,i, cSF,i),        c2.SF,2i=(cSF,i, −cSF,i).        
The chips of a channel code cSF,i modulate, at the rate D=3.84 Mchip/s in the case of UMTS, symbol trains whose rate is D/SF=2k−L.D, that is to say the spreading factor equals SF=2L−k. The symbols in question may be complex symbols each comprising two signed bits (of value ±1) corresponding to an I pathway and to a Q pathway.
The OVSF codes allocated to various channels are chosen in such a way as to be globally orthogonal to one and the same sender. With the code tree of FIG. 1, two codes having the same spreading factor are always orthogonal, the sum of the chip-to-chip products being zero. Two codes with spreading factors 2L−k and 2L−k′ are orthogonal if, after they have modulated any two sequences of signed bits of respective rates 2k−L.D and 2k′−L.D, the resulting chip sequences are orthogonal. With the tree arrangement of FIG. 1, this amounts to saying that two channel codes are orthogonal if and only if they do not belong to one and the same branch of the tree, going from the root c1,1 to a leaf cL,i. The selecting of the codes obeys this constraint globally: the set of channel codes used at the same instant by the sender is such that two codes are not found on the same branch. This allows the receivers to discriminate between the channels relevant to them.
In a system using such codes, it is judicious to allocate the codes in such a way that two communications which are close by, and hence liable to interfere with one another, use orthogonal codes, so as to improve the quality of these communications.
SDMA (Space Division Multiple Access) radiocommunication systems are moreover known. Such systems comprise base stations whose antenna, sometimes dubbed an intelligent or “smart” antenna, comprises a plurality of radiating elements controlled jointly so as to send to or receive from a terminal. The radiating elements may send or receive simultaneously with a weighting of the signal by a complex coefficient whose argument depends on an angular direction relating to the terminal with which the antenna is exchanging signals.
EP-A-1 026 911 describes, in respect of a radio relay having multiple send/receive capabilities in various predetermined geographic sectors, the apportioning of the codes available into several subsets, some being scheduled for use by fixed terminals situated in guard sectors, positioned between base sectors, and the others being scheduled for use by fixed terminals situated in the base sectors.
The apportioning of the codes which is disclosed in that document is well-tailored to a network where the communication terminals are fixed and where the subsets of codes may be allocated once and for all or over long periods of time, by geographical zone. On the other hand, a problem arises in radiocommunication systems where the terminals are mobile. The transposing of this scheduling scheme to radiocommunications with mobiles is problematic since it becomes necessary to make allowance for the mobility of the terminals, and in particular the case where they change sector, so as to ascertain the subset from which communication codes should be chosen for them. In fact, in a mobile radio context, each of the predetermined sectors would correspond to a distinct cell requesting its own signalling resources, involving a greater loading of the equipment in respect of the procedures for cell selection/reselection, handover, etc. Various mobile radio systems, such as UMTS, are furnished with other means for distinguishing between various cells, and the scheme of EP-A-1 026 911, intended for fixed terminals, would afford them nothing save greater complexity and lesser availability of the OVSF codes.
An object of the present invention is to propose a dynamic allocation of the resources in a space division radiocommunication system comprising mobile terminals.
Another object of the invention is to allow an allocation of codes taking account of the codes already allocated and of the actual interference measured.
Yet another object of the invention is to allow an allocation of codes taking into account the sense and/or the speed of movement of the mobile terminals.